Temperatures will climb more than 15 degree above average in parts of Victoria, where Melbourne is tipped to hit 42 degrees and Mildura 46 degrees. This would be Melbourne's hottest day in three years, while Mildura may get close to Victoria's January record of 47.2 degrees.

Other sweaty capital cities on Friday will include Canberra (37C), Hobart (36C) and Adelaide (35C), while temperatures could also reach close to 40 degrees in western Sydney.

The hot air and gusty winds will cause raised dust and elevated fire danger in southeastern Australia on Friday, with severe to extreme fire danger ratings likely in parts of Victoria and SA.

As the trough crosses the nation's southeast, the hot northerly winds will be replaced by much cooler, blustery southwesterlies, dropping temperatures by 15-20 degrees in some places.

This cool change should reach Adelaide in the morning, cross Tasmania and Victoria during the afternoon and evening and progress into southern NSW on Friday night and Saturday morning.

In addition to the heat, showers and thunderstorms are possible ahead of and with the passage of the trough, which could start new fires or exacerbate existing ones.

Refer to your state's fire service website for the latest information and warnings.